1. Field of the Invention
This invention relates to an optical atmospheric link system, and more particularly to an optical atmospheric link devices which irradiate optical beams in atmosphere to transmit desired data therebetween.
2. Description of the Prior Art
Heretofore, an optical atmospheric link system for irradiating optical light beams in the atmosphere to transmit desired data was provided. Two such optical atmospheric link devices may be installed on the roofs of buildings which are spaced, for example, several kilo-meters [km] from each other, and desired data transmitted therebetween.
In order to transmit data correctly between the two optical atmospheric link devices, an optical beam irradiated from a signal transmitting device of each of the apparatus should be correctly applied to a signal receiving device of the other. In a typical optical atmospheric link system, the optical systems within the signal transmitting device and the signal receiving device have a correcting optical device, including a mirror and a convex lens. The mirror and the convex lens are moved suitably to adjust the direction of radiation (i.e., position of incidence) of the light beam.
Hence, in installation of the optical atmospheric link devices, the devices are first roughly positioned, using a telescope or the like. Thereafter, an optical beam is transmitted through the atmosphere from one of the devices to the other. Then, the mirrors and a convex lens as in the devices are adjusted such that the signal receiving device receive the light beam correctly. This adjusting operation is carried out for the two devices alternately and repeatedly, until the optical beam is transmitted in a correct direction and is received at a correct position.
However, the optical axis may be shifted by external disturbances such as wind and vibration. Therefore, the signal receiving device is provided with an incident position correcting servo system which monitors the position of incidence of the optical beam transmitted from the signal transmitting device of the other optical atmospheric link device at all times and, when the position of incidence is shifted, drives the mirror and the convex lens to correct it.
In the incident position correcting servo system, the mirror is used to deflect the reflected optical axis in a predetermined direction. The mirror movement is driven, for instance, by a piezo-electric actuator. Furthermore, the convex lens is moved in a direction substantially perpendicular to the optical axis, so as to deflect the optical axis in a predetermined direction. The lens movement is driven by a plurality of electric motors.
In the incident position correcting servo device, the mirror is driven by the piezo-electric actuator in a high response mode; however, since the actuator is small in vibration amplitude, the amount of displacement of the mirror is small. Hence, if the mirror is displaced from the center of displacement only in one direction, then the direction of displacement which can be controlled is limited, as a result of which the shift in the position of incidence by an external disturbance cannot be corrected.
On the other hand, the convex lens, being driven in a large amplitude mode by the motor, can respond to larger magnitude disturbances; however, it cannot respond to a disturbance quickly because the motor itself is low in response characteristic. Even if the mirror and the convex lens are operated in combination, each performs its control operations independently of the other, and therefore the incident position correcting servo device is still insufficient in practical use.